Porous paper or paperboard is usually surface sized, or blade coated, in order to close the surface and hence to enhance the surface strength, optical properties or improve e.g. the printability.
However, impregnation or surface sizing of dense webs such as thin films made of cellulosic nanofibers or microfibrillated cellulose, with basis weight of around 10-30 g/m2, is almost impossible since the surface is closed and not capable of absorbing surface sizing chemicals. In fact, a dense film with grammage of approximately 30 g/m2, may have relatively good barrier properties measured as the oxygen transmission rate (OTR) particularly at 50% RH or below (see e.g. Aulin et al., Oxygen and oil barrier properties of microfibrillated cellulose films and coatings, Cellulose (2010) 17:559-574, Lavoine et al., Microfibrillated cellulose—Its barrier properties and applications in cellulosic materials: A review, Carbohydrate polymers 90 (2012) 735-764, Kumar et al., Comparison of nano- and microfibrillated cellulose films, Cellulose (2014) 21:3443-3456).
However, the surface treatment or impregnation of such a film at high speeds, where the contact times between coating or impregnation and drying are short, is very difficult. Without being bound to any theory, an extended impregnation nip and longer contact times will probable facilitate the film swelling, diffusion and penetration of both water and the applied chemicals. On the other hand, a prolonged impregnation step might also weaken the inter-fibrillar and cellulose interactions which lead to a weakened web, which then might break. The use of wetting chemicals, or chemicals that enhance the permeability might also be an option but in many applications there is a need to limit the amount of functional chemicals.
Another challenge of coating a nonporous web is to ensure that there are enough adhesion forces formed between the base substrate and the applied coating. In this respect, both mechanical interlocking and chemical or physical interactions are important for avoiding release of the applied coating.
Thus, surface sizing, film press sizing or other types of impact coating processes are not efficient on a very dense substrate and oftentimes lead to a structured substrate, i.e. a clear difference between top, middle and back layer.
By using e.g. rotogravure or reverse gravure or flexography, it is possible to apply thin or low amounts, of coating to the web. However, these methods usually put limitations on coating weights and machine widths. When the roll length exceeds a certain length, problems with the web profile (coat weight variations in cross-machine direction) may occur.
There is thus a need for a method of surface sizing dense films or webs, without causing any web breaks. Moreover, the method should be applicable for a high speed processes and wider paper machines.